This section provides background information related to the present disclosure which is not necessarily prior art.
For centuries, the pyrometallurgical production of metal from ores or ore concentrates has resulted in the creation of slags and drosses as a by-product. The compositions of these slags and drosses vary, but they are typically a glassy or vitreous material with residual metal values in oxide, sulfide, and even elemental form. Because many of these slags and drosses can contain heavy metals such as lead, they generally cannot be put to use as a construction material or employed for other purpose. In older slags and drosses the metal values can be quite high, and even in more recent slags significant metal values are high despite improvements in metal recovery technologies.
Because of the residual metal values, slags and drosses are often the subject of environmental regulation, as they are in the United States. In the United States most slags are deemed hazardous waste as defined RCRA (40 CFR Part 261), but benefit from the Bevill exclusion, an amendment to the Resource Conservation and Recovery Act (RCRA) which allows slags to be stored at certain permitted locations. However there are continuing obligations of monitoring and management of these permitted locations If these slags could be made to pass the EPA's Toxicity Characteristic Leaching Procedure (TCLP) according to EPA Test Method 1311, then they would not qualify as hazardous waste under classifications D004 through D052 as defined RCRA (40 CFR Part 261), and they could be removed and used (for example in building and roadway construction) and the land where they had been stored reclaimed for other uses.
Despite these reasons and the substantial economic value of the metals trapped in slag piles around the world, little has been done to recover these metals because conventional wisdom was that slag had to be reheated in order to release a significant fraction of the metals from the glassy matrix. The most common slag treatment technology is slag fuming. The slag is melted in a furnace at a very high temperature and the vaporized metals are recovered. To increase the efficiency of the separation, the time of the operation is increased to allow enough time for any vaporized metal to pass through the slag layer and go with the vaporized metals. This approach, in any case, generates another slag. To increase the metal extraction in fuming, chemical additions to the melt are also required which add to the volume of the final slag. This process in very energy intensive, and does not necessarily result in complete recovery of the metal values, which means that the resulting slag may still be considered a hazardous material.